Filled polypropylene articles have been observed to exhibit undesirable odors, particularly upon heating. In this respect, see U.S. Pat. No. 5,023,286 to Abe et al., wherein phenolic antioxidants are suggested to control the odor problem. Other polypropylene compositions may be found in U.S. Pat. No. 4,734,450 to Kawai et al.; U.S. Pat. No. 5,045,369 to Kobayashi et al.; U.S. Pat. No. 5,300,747 of Simon; U.S. Pat. No. 5,439,628 of Huang and U.S. Pat. No. 4,933,526 of Fisher et al.
This invention relates to disposable, mineral-filled polypropylene microwaveable containers having suitable food contact compatible olfactory properties including cups, trays, souffle dishes, lids, plates, bowls, and related articles of manufacture useful for preparation, storage, delivery, and serving of food, wherein convenience and low cost are of paramount importance. Nevertheless, suitable food contact compatible olfactory properties, appearance, and tactile characteristics of the plate, container, etc., are important for consumer preference. The suitability of these disposable articles of manufacture for microwave cooking, or heating of food, has an important place in today""s marketplace. Both the commercial and retail market components need an aesthetically pleasing microwaveable, disposable, rigid and strong container, plate, or cup, and related articles of manufacture which also have suitable food contact compatible olfactory properties. These disposable microwaveable containers and plates further exhibit a melting point of no less than about 250xc2x0 F., the containers or plates being dimensionally stable and resistant to grease, sugar and water at temperatures up to at least 220xc2x0 F. and exhibiting sufficient toughness to be resistant to cutting by serrated polystyrene flatware.
Microwaveable, disposable, rigid and strong containers and plates having suitable food contact compatible olfactory properties have been prepared. These disposable and microwaveable articles of manufacture exhibit (a) suitable food contact compatible olfactory properties; and (b) a melting point of not less than 250xc2x0 F., suitably 250xc2x0 F. to 330xc2x0 F. In preferred embodiments these articles of manufacture exhibit a micronodular surface on the side coming in contact with food. These microwaveable, food contact compatible containers and plates are dimensionally stable and resistant to grease, sugar and water at temperatures of at least 220xc2x0 F. and are of sufficient toughness to be resistant to cutting by serrated polystyrene flatware. The containers and plates of this invention answer a long felt need for products which can withstand the severe conditions of a microwave oven when common foods such as beans and pork, pancakes with syrup, pepperoni pizza, and broccoli with cheese are microwaved during food cooking and reconstituting processes.
It has been found in accordance with the present invention that mineral-filled polypropylene food contact articles such as bowls or plates exhibit superior olfactory characteristics when prepared including a basic organic or inorganic compound.
There is provided in a first aspect of the present invention a method of preparing a microwaveable, mineral-filled polypropylene food contact article including the steps of: (a) preparing a melt-compounded composition with from about 40 to 90 percent by weight of a polypropylene polymer; from about 10 to about 50 percent by weight of a primary mineral filler and an effective odor-reducing amount of a basic organic or inorganic compound, the melt-compounded compositon exhibiting an odor index of less than about 0.75; (b) extruding the aforesaid melt comounded composition into sheet form; and (c) forming the food contact article from the sheet, wherein the basic organic or inorganic compound is operative to reduce undesireable odors in the melt-compounded composition to the aforesaid odor index value of 0.75 or less. The primary filler is mica, clay, a siliceous material, ceramics, glass, a sulfate mineral, or mixtures thereof.
Typically, the primary mineral filler is mica, talc, kaolin, bentonite, wollastonite, milled glass fiber, glass beads (solid or hollow), silica, or silicon carbide whiskers or mixtures thereof. We have discovered that when polypropylene is melt-compounded with acidic-type minerals the resulting mixture has a higher odor index (offensive odors) that would disqualify them from use in food service products.
Acidic type fillers such as mica; natural clay minerals such as kaolinite, bentonite, attapulgite, montmorillonite, clarite, or fuller""s earth; and silica are particularly detrimental in generating odor compounds when processed under high shear and high temperature conditions experienced during twin screw compounding. We have found that changing the compounding process and adding a basic component to the primary acidic filler allows the production of low odor index compounds. The reason for this effect is unknown since the fundamental cause of the degradation in polypropylene may be due, in part, to catalysis effects caused by impurities in the mineral as well as its acidic or basic nature. In this regard, the addition of CaCo3 to talc is beneficial whereas, it may be unnecessary when wollastonite is used as the primary filler.
The preferred primary fillers are mica, talc, kaolin, bentonite, milled glass fibers, and wollastonite or mixtures thereof. Of these milled glass fibers and wollastonite are basic in nature and may not necessarily require the addition of a secondary basic component.
As noted above, suitable mineral fillers include mica, talc, kaolin, bentonite, wollastonite, milled glass fiber, glass beads (hollow or solid), silica whiskers, silicon carbide whiskers and mixtures thereof as well as the mineral fillers recited herein, whereas the basic organic or inorganic compound is generally the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides including mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of the foregoing. More specifically, the basic organic or inorganic compound may be selected from the group consisting of: calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium citrate, potassium citrate, calcium stearate, potassium stearate sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the above. Furthermore, hydroxides of the metals and alkaline earth elements recited above may be utilized.
Where a basic inorganic odor suppressing compound is chosen, generally such compound is selected from the group consisting of calcium carbonate, sodium carbonate, potassium carbonate, barium carbonate, aluminum oxide, sodium silicate, sodium borosilicate, magnesium oxide, strontium oxide, barium oxide, zeolites, sodium phosphate, potassium phosphate, magnesium phosphate, mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures of one or more of the basic inorganic compounds set forth above. The amount of a basic inorganic compound is generally from about 2 to 20 weight percent, but is usually from about 5 to about 15 weight percent of the article. Most preferably the basic inorganic compound selected is calcium carbonate; typically present from about 5 to about 20 weight percent.
Where an organic compound is chosen, it is typically selected from the group consisting of sodium stearate, calcium stearate, potassium stearate, sodium citrate, potassium citrate, and mixtures of these where the amount of such compound is from about 0.5 to about 2.5 weight percent of the article.
Typically, microwaveable articles produced in accordance with the present invention exhibit an odor index of less than about 0.75; preferably less than about 0.6; with a practical lower limit being 0.1 or so.
As shown below in connection with microwaveability testing, and summarized in Table 20, competing commercial polystyrene type plates cannot withstand the high temperatures generated in the microwave oven during food contact and either significantly warp or deform when the aforementioned food products were heated on them. Under the usual microwaving conditions with high grease content foods, the prior art plates tend to deform and flow to the point where parts of the plate become adhered to the inside of the microwave oven. For disposable plates and containers, having suitable food contact olfactory properties, appearance and feel are important attributes. Another significant property of the containers and plates of this invention is their cut resistance. These rigid articles of manufacture are of sufficient toughness to be resistant to cutting by serrated polystyrene flateware. In normal usage they are also resistant to cutting by regular metal flatware.
Whereas any microwaveable article may be produced in accordance with the invention, most typically the article is a bowl or a plate suitable for serving food at a meal. Preferred articles are thermoformed and include a micronodular food contact surface. Micronodular food contact surfaces are produced by thermoforming a sheet into the article which has been extruded optionally with at least one matte roll and by vacuum thermoforming the sheet by applying vacuum opposite to the surface where the micronodular surface is desired. Most typically the micronodular surface will have a surface gloss of less than about 35 at 75xc2x0 as measured by TAPPI method T-480-OM 92. Articles also will typically have a Parker Roughness Value of at least about 12 microns.
While any suitable polypropylene polymer may be used, the polypropylene polymers are preferably selected from the group consisting of isotactic polypropylene, and copolymers of propylene and ethylene wherein the ethylene moiety is less than about 10% of the units making up the polymer, and mixtures thereof. Generally, such polymers have a melt flow index from about 0.3 to about 4, but most preferably the polymer is isotactic polypropylene with a melt-flow index of about 1.5. In particularly preferred embodiments, the melt-compounded composition from which the resultant extruded sheet is formed into articles further includes a polyethylene component and titanium dioxide. The polyethylene component may be any suitable polyethylene such as HDPE, LDPE, MDPE, LLDPE or mixtures thereof.
The various polyethylene polymers referred to herein are described at length in the Encyclopedia of Polymer Science and Engineering (2d Ed.), Vol. 6; pp: 383-522, Wiley 1986; the disclosure of which is incorporated herein by reference. HDPE refers to high density polyethylene which is substantially linear and has a density of generally greater that 0.94 up to about 0.97 g/cc. LDPE refers to low density polyethylene which is characterized by relatively long chain branching and a density of about 0.912 to about 0.925 g/cc. LLDPE or linear low density polyethylene is characterized by short chain branching and a density of from about 0.92 to about 0.94 g/cc. Finally, intermediate density polyethylene (MDPE) is characterized by relatively low branching and a density of from about 0.925 to about 0.94 g/cc. Unless otherwise indicated these terms have the above meaning throughout the description which follows.
The microwaveable articles according to the invention typically exhibit melting points from about 250 to about 330xc2x0 F. and include mica or other primary fillers in amounts from about 20 to about 35 weight percent. Most preferably mica is present at about 30 weight percent, and calcium carbonate is present from about 8 to about 12 weight percent.
It has been found that C8 and C9 organic ketones correlate well with or are associated with undesirable odors in polypropylene/mica compositions. Accordingly, it is preferred that articles in accordance with the invention are substantially free from volatile C8 and C9 organic ketones. In order to avoid undesirable odors, articles in accordance with the invention are preferably prepared from a melt-compounded polyolefin mica composition which is prepared at a process melt temperature of less than about 425xc2x0 F.; with below about 400xc2x0 F. being even more preferred. Optionally, the melt processed polyolefin/mineral composition is melt-compounded in a nitrogen atmosphere.
In another aspect of the invention, there is provided a thermoformed, mineral-filled polypropylene food contact article formed from a melt-compounded composition comprising from about 40 to about 90 percent by weight of a polypropylene polymer, from about 10 to about 50 percent by weight of a primary mineral filler and an effective odor-reducing amount of a basic organic or inorganic compound operative to impart an odor index of less than about 0.75 to said melt-compounded composition.
Preferably the inventive articles are prepared from a melt-compounded polyolefin/mica composition prepared by way of a low temperature compounding process.
A preferred low temperature compounding process used for producing mineral-filled polypropylene melt-compounded compositions with an odor index of less than about 0.75 including a basic odor suppressing agent in accordance with the invention with from about 40 to about 90 percent by weight of a polypropylene polymer includes the sequential steps of: (a) preheating a polypropylene polymer while maintaining the polymer below a maximum temperature of about 370xc2x0 F. and preferably below 350xc2x0 F. and more preferably below a maximum of about 260xc2x0 F.; but suitably above about 240xc2x0 F.; followed by; (b) admixing mineral filler to said preheated polymer in an amount from about 10 to about 50 percent weight based on the combined weight of the resin and primary filler and maintaining the mixture below about 425xc2x0 F.; followed by, (c) extruding the mixture. Polymer may be melted exclusively through the application of shear, or the shear may be supplemented through heating by infrared radiation or ordinary heating coils or performed externally to the mixing chamber. Preferably, the basic odor suppressing agent is added simultaneously with the mineral filler.
It is desirable to keep the duration of the step of admixing mineral filler and a basic odor suppressant agent to the mixture relatively short so as not to generate compounds which cause odor and to preserve the particle size and aspect ratio of the mineral filler. Accordingly, the step of admixing the mineral filler should be no more than about five minutes with the duration of the admixing step of less than about three minutes being even more preferred. Any suitable means may be used to carry out the sequential process in accordance with the invention, however, the process is normally carried out in a batch mode in a mixing chamber provided with a pair of rotating rotors in an apparatus referred to in the industry as a Banbury type mixer. One may choose to use a twin screw extruder or a Buss kneader to practice the inventive process if so desired, provided that appropriate elements are used to minimize shear heating.
Thermoforming is typically conducted at a sheet temperature of from about 260xc2x0 to about 310xc2x0 F., and more preferably at a temperature of from about 280xc2x0 to about 300xc2x0 F.
There is provided in a still further aspect of the invention a crack-resistant, thermoformed food contact article having a wall thickness ranging from about 10 to about 80 mils consisting essentially of from about 40 to about 90 weight percent of a polypropylene polymer, from about 10 to about 50 percent by weight of a mineral filler, from about 1 to about 15 percent by weight polyethylene, from about 0.1 to about 5 weight percent titanium dioxide and optionally including a basic organic or inorganic compound. The basic compound is, generally speaking, the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides, including mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures thereof. A particularly preferred article is where the basic organic or inorganic compound is calcium carbonate which is present in an amount of from about 5 to about 20 weight percent.
Polyethylene is more typically present from about 2.5 to about 15 weight percent, preferably from about 4 to about 5 weight percent of the crack resistant article.
Titanium dioxide is included in various amounts, from about 0.1 to about 3 percent by weight being typical; from about 0.25 to 2 percent titanium dioxide may be included. Preferably, titanium dioxide is included in at least 0.5 percent by weight.
The caliper, or wall thickness, of the articles is usually from about 0.010 to about 0.050 inches or from about 10 mils to 50 mils. A caliper of from about 15 to 25 mils is most typically employed.
While any suitable polypropylene polymer may be employed, the most preferred polymer is isotactic polypropylene having a melt index in the range of from about 0.3 to 4, with a melt index of about 1.5 being typical. The polyethylene employed may be HDPE, LLDPE, LDPE or MDPE, mixtures thereof or a polyethylene with bimodal molecular weight distribution. Polypropylene is sometimes referred to hereafter as xe2x80x9cPPxe2x80x9d.
The inventive compositions from which the crack resistant articles are made do not include coupling agents such as maleic anhydride containing polypropylene as further described herein, but may optionally include other components which do not alter the basic and novel characteristics of the crack-resistant plates. For example, nucleants such as sodium benzoate in amounts detrimental to crack resistance are to be avoided.
In a still further aspect of the invention there is provided a method of making a microwaveable mineral-filled polypropylene food contact article comprising preparing a melt-compounded composition comprising from about 40 to about 90 percent by weight of a polypropylene polymer and from about 10 to about 50 percent by weight of a mineral filler and optionally an effective amount of an odor-reducing compound. The melt-compounded composition exhibits a relative aroma index, relative to a corresponding composition consisting essentially of polypropylene and mica of less than about 0.75. The composition is extruded into a sheet and formed into a suitable food contact article. Preferably, the article consists essentially of polymer and mineral filler and excludes such components as excess anti-oxidants and the like.